Page description data processing apparatus, page description data processing method, and recording medium

ABSTRACT

It is confirmed whether or not page description data contain an object for stroking a path. If the page description data contain the object for stroking a path, then it is confirmed whether or not a curved path construction operator is contained in the object. If the curved path construction operator is contained in the object, line segments of a polygonal path which is approximated to a curved path of the curved path construction operator are determined, and the curved path construction operator is replaced with a plurality of straight path construction operators which correspond respectively to the line segments of the polygonal path.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2009-270710 filed on Nov. 27, 2009, ofwhich the contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a page description data processingapparatus, a page description data processing method, and a recordingmedium for performing a particular processing sequence on pagedescription data described by a page description language (PDL), therebyto convert the page description data into more robust page descriptiondata (hereinafter referred to as “robustized page description data”).

2. Description of the Related Art

In recent years, DTP processes for setting characters and imagesgenerated by the operator with a computer into an electronic page usingDTP (DeskTop Publishing) application software installed in the computerhave been widely used in the printing and platemaking fields.

The DTP application software generates page description data whichexpress images of respective pages based on elements such as charactersand images edited by the operator.

The page description data are vector data independent of the resolutionof an output machine such as a printer, a platesetter, or the like, andcannot be output per se from the output machine. Therefore, the pagedescription data are rasterized by an RIP (Raster Image Processor) intoraster image data comprising a cluster of dots representative ofelements such as characters and images in pages.

When the raster image data are supplied to the output machine such as aprinter, a platesetter, or the like, the output machine outputs a hardcopy or a printing plate carrying an image based on the raster imagedata (see Japanese Laid-Open Patent Publication No. 2005-070957).

PDF (Portable Document File) version 1.3, which is one type of pagedescription data, incorporates a plurality of processing schemes(hereinafter referred to as “line join styles”) that are applied tojoints in stroking a plurality of connected paths. The term “path”refers to a path which has no line width by itself and whichinterconnects a start point and an end point. The term “stroke” refersto a process of applying a certain line width to a path.

Of the line join styles incorporated in PDF version 1.3, “miter join” isa process of extending the outer edges of two line segments until theymeet at a certain angle, as in a picture frame. Another line join stylecalled “bevel join” is a process of squaring off the end points of twoline segments and filling a space defined jointly by the square endswith a triangle. For details, reference should be made to PDF Reference,2nd edition, Adobe Portable Document Format Version 1.3, 1st edition,1st print, published July 2000, Author: Adobe Systems, Publisher:Pearson Education, ISBN0-201-61588-6, pages 135-136, 140-141.

According to the PDF specifications, it is prescribed that if the angleat which two line segments cross each other is equal to or greater thana certain value (miter limit), then the miter join is applied, and ifthe angle is smaller than the miter limit, the bevel join is applied.Therefore, if the line segments cross each other at an acute angle, thenthe corner formed by the crossing line segments is squared off toprevent itself from protruding excessively, with the result that theyhave an improved image quality, particularly if they produce a characterimage.

However, even the PDF specifications are susceptible to an unexpectedproblem which is experienced in a process of rasterizing a line shapethat is made up, as shown in FIG. 8A of the accompanying drawings, of acurved path P_(C) having a start point A₁ and an end point A₂ and astraight path P_(L) having a start point A₂ and an end point A₃. Theproblem will be described below while the rasterizing process will beexplained.

As shown in FIG. 8B of the accompanying drawings, an object O_(C) (seeFIG. 8A) strokes the curved path P_(C) (indicated by the solid line inFIG. 8A), and an RIP approximates the curved path P_(C) with a polygonalpath P_(L1) (indicated by the broken line in FIG. 8B), made up of aplurality of line segments. The approximated result depends on thecharacteristics inherent in the output machine, such as an outputresolution thereof, and the software version of the RIP. In FIG. 8B, thecurved path P_(C) comprises a curve A₁A₂ divided into two line segmentsby a joint J₁.

Then, the shapes of the outer edges of line segments having a certainline width are determined along and over a central line that isrepresented by the polygonal path P_(L1). The angle ∠J₁A₂A₃ formedbetween a line segment J₁A₂ and a line segment A₂A₃ varies depending onthe position of the joint J₁ that has been determined by the aboveapproximation. Consequently, it is impossible to predict which one ofthe miter join and the bevel join is to be applied as the line joinstyle at the point A₂.

If the angle ∠J₁A₂A₃ in FIG. 8B is smaller than the miter limit, thenthe corner of the point A₂ is processed into a bevel join, as shown inFIG. 8C. As a result, a stroked area 4 is produced which is devoid of anapex (cut-off area 2) at the corner of the point A₂.

In FIG. 8D of the accompanying drawings, the curve A₁A₂ is divided intothree line segments by joints J₂, J₃, and is approximated by a polygonalpath P_(L2), indicated by the broken line.

The angle ∠J₃A₂A₃ between a line segment J₃A₂ and a line segment A₂A₃shown in FIG. 8D is greater than the angle ∠J₁A₂A₃ shown in FIG. 8B. Ifthe angle ∠J₃A₂A₃ is equal to or greater than the miter limit, then thecorner of the point A₂ is processed into a miter join, as shown in FIG.8E of the accompanying drawings. As a result, a stroked area 6 isproduced which has an apex at the corner of the point A₂.

As shown in FIGS. 8B and 8D, since the angle at which the curved pathP_(C) and straight path P_(L) join each other varies depending on thenumber of line segments into which the curve A₁A₂ is divided, thecut-off area 2 (see FIG. 8C) may be irregularly produced. The samephenomenon may occur with respect to a line shape that is made up ofjoined objects each stroking a curved path.

As described above, when curved paths are approximated by polygonalpaths depending on the characteristics inherent in the output machine,such as an output resolution thereof, and the software version of theRIP, since the angle at which the paths join each other is indefinite,they may be converted into an unexpected stroked shape. Accordingly,when image data representing a precipitous line shape are rasterized,the rasterized image data may possibly lead to unexpected printdifficulties whose causes are difficult to identify.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a page descriptiondata processing apparatus, a page description data processing method,and a recording medium which make it possible to prevent printdifficulties from occurring in a process of rasterizing image datarepresenting a precipitous line shape.

A page description data processing apparatus, a page description dataprocessing method and a recording medium according to the presentinvention operate by confirming whether or not page description datacontain an object for stroking a path or not, confirming whether or nota curved path construction operator is contained in the object, if thepage description data contain the object for stroking a path, anddetermining line segments of a polygonal path which is approximated to acurved path of the curved path construction operator and replacing thecurved path construction operator with a plurality of straight pathconstruction operators which correspond respectively to the linesegments of the polygonal path, if the curved path construction operatoris contained in the object. In this way, page description data isconverted into more robust page description data (referred to as“robustized page description data”).

Preferably, upon replacing the curved path construction operator, theline segments of the polygonal path are determined such that anapproximation error with respect to the curved path falls within apredetermined range.

Since the curved path construction operator is replaced with thestraight path construction operators which correspond respectively tothe line segments of the polygonal path, no calculation process iscarried out to approximate a curved path by a polygonal path in arasterizing process carried out by an RIP. Therefore, it is possible toprevent the angle at which paths join each other from becomingindefinite in the page description data. The page description data arethus prevented from leading to print difficulties in a process ofrasterizing image data representing a precipitous line shape.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an entire publishing system according to anembodiment of the present invention;

FIG. 2 is a functional block diagram of a page description dataprocessing apparatus of the publishing system shown in FIG. 1;

FIG. 3 is a diagram showing a path represented by a cubic Bezier curvethat is generated by a curve stroke operator;

FIG. 4 is a flow chart of an operation sequence of the page descriptiondata processing apparatus shown in FIG. 2;

FIG. 5 is a flow chart of a process of replacing a path constructionoperator;

FIG. 6 is a diagram illustrative of a process of determining a polygonalpath which is approximated to a path represented by a cubic Beziercurve;

FIGS. 7A through 7C are diagrams illustrative of a rasterizing processaccording to the embodiment of the present invention; and

FIGS. 8A through 8E are diagrams illustrative of a rasterizing processaccording to a comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A publishing system which incorporates a page description dataprocessing apparatus, a page description data processing method, and arecording medium according to an embodiment of the present inventionwill be described in detail below with reference to the accompanyingdrawings.

FIG. 1 is a block diagram of an entire publishing system 10 according toan embodiment of the present invention.

The publishing system 10 is arranged to perform a prepress process, aprinting process, and a bookbinding process (not shown).

The prepress process includes a DTP computer 12, a page description dataprocessing apparatus 14 such as a personal computer or the like, an RIP16, a printer 20, and a platesetter 22.

The DTP computer 12 generates page description data Dp which expressimages of respective pages based on elements such as characters andimages edited by the operator.

The page description data processing apparatus 14 checks the contents(attributes) of the page description data Dp output from the DTPcomputer 12, performs a particular processing sequence on the pagedescription data Dp having a certain attribute based on the checkedcontents, and generates the processed page description data Dp asrobustized page description data Dp′ or outputs page description data Dpfree of the certain attribute. The processing function of the pagedescription data processing apparatus 14 may be incorporated in the DTPcomputer 12, so that the page description data processing apparatus 14can be dispensed with.

The RIP 16 converts the page description data Dp or the robustized pagedescription data Dp′ output from the page description data processingapparatus 14 into raster image data Dr in C, M, Y, K.

Based on the raster image data Dr, the printer 20 prints a proof 18 as ahard copy.

If the proof 18 printed by the printer 20 is judged as acceptable by theoperator, then the platesetter 22 generates and outputs printing platesPP in C, M, Y, K from the raster image data Dr output from the RIP 16after the operator has turned on the start switch of the platesetter 22.

The printing process includes a printing press 24. The printing press 24has printing plates PP in C, M, Y, K mounted therein which carryrespective inks in C, M, Y, K and transfer them to a sheet of paper,thereby producing a print 26 in multiple colors (four colors).

FIG. 2 is a functional block diagram of the page description dataprocessing apparatus 14. The page description data processing apparatus14 has functions that are performed when a CPU 14 a (see FIG. 1) thereofexecutes a program stored in a ROM 14 b (see FIG. 1) thereof.

The program may be recorded in a computer-readable recording medium suchas the ROM 14 b, a hard disk, a CD-ROM or the like, and the programrecorded in the recording medium may be read and run by a computersystem. The computer system includes an OS and hardware such as a CPUand peripheral devices. The computer-readable recording medium alsoincludes a medium for dynamically holding programs for a short period oftime, such as a communication line for transmitting programs via anetwork such as the Internet or the like or a communication channel suchas a telephone line, or a memory for holding programs for a certainperiod of time such as a volatile memory in a computer system whichserves as a server or a client in a network environment.

The page description data processing apparatus 14 generates robustizedpage description data Dp′ by analyzing page description data Dp suppliedvia an input interface (input I/F) 32 or generates unchanged pagedescription data Dp, and outputs the robustized page description dataDp′ or the unchanged page description data Dp via an output interface(output I/F) 34.

The page description data processing apparatus 14 comprises, in additionto the input and output interfaces 32, 34, an object confirming unit 36involved in the analyzing process, a curved path construction operatorconfirming unit 42, and a path construction operator replacer 44, as itsfunctions. The object confirming unit 36 comprises a structuralanalyzer, not shown, a path object confirming unit 38, and apath-painting operator attribute confirming unit 40.

The term “path object” used herein refers to an arbitrary shape made upof a straight line, a rectangle, and a curve, e.g., a cubic Beziercurve.

The term “path-painting operator” used herein refers to an operator forending a path object, having an attribute as either an operator forapplying a line width to a current path to paint a line (hereinafterreferred to as “stroking operator” for illustrative purposes) or anoperator for filling a closed space generated by a current path(hereinafter referred to as “filling operator” for illustrativepurposes).

The term “path construction operator” used herein refers to an operatorfor defining the physical shape of a path. According to the PDF, thereare defined seven operators “m”, “l”, “c”, “v”, “y”, “h”, and “re”.

The term “curved path construction operator” used herein refers to anoperator for adding a curve to a current path, among the pathconstruction operators. According to the PDF, there are defined threeoperators “c”, “v”, and “y”, each for adding a cubic Bezier curve.

The term “straight path construction operator” used herein refers to anoperator for adding a straight line to a current path, among the pathconstruction operators. According to the PDF, there is defined oneoperator “l”.

FIG. 3 is a diagram showing a cubic Bezier curve that is generated bythe operator “c”. The cubic Bezier curve, denoted by 50 in FIG. 3, isdefined by two terminal points, i.e., start and end points, P₀ (x₀, y₀),P₃ (x₃, y₃) and two control points P₁ (x₁, y₁), P₂ (x₂, y₂).

The cubic Bezier curve 50 is represented by coordinates P(x(t), y(t))where t is a variable, and is generated between the start point P₀ andthe end point P₃ by varying the variable “t” from 0 to 1. Thecoordinates P(x(t), y(t)) of a point P on the curve 50 are calculated bythe following expressions (1), (2):x(t)=(1−t)³ x ₀+3t(1−t)² x ₁+3t ²(1−t)x ₂ +t ³ x ₃  (1)y(t)=(1−t)³ y ₀+3t(1−t)² y ₁+3t ²(1−t)y ₂ +t ³ y ₃  (2)

Since x(0)=x₀, y(0)=y₀, x(1)=x₃, and y(1)=y₃ from the equations (1),(2), when the variable “t” changes from 0 to 1, the point P on the cubicBezier curve 50 moves continuously from the start point P₀ to the endpoint P₃ along the direction indicated by the arrow D. A line segmentP₀P₁ indicated by the broken line is a line tangential to the cubicBezier curve 50 at the start point P₀, and a line segment P₂P₃ indicatedby the broken line is a line tangential to the cubic Bezier curve 50 atthe end point P₃.

The publishing system 10 is basically constructed as described above.Operation of the page description data processing apparatus 14 of thepublishing system 10 will be described below with reference to aflowchart shown in FIG. 4.

In step S1 shown in FIG. 4, the page description data processingapparatus 14 reads page description data Dp, page by page, which areoutput from the DTP computer 12.

In step S2, the structural analyzer, not shown, analyzes the structureof the page description data Dp and extracts an object contained in thepages. Thereafter, the path object confirming unit 38 confirms whether apath object is contained in the extracted object or not. If a pathobject is not contained in the extracted object, then the path objectconfirming unit 38 outputs the page description data Dp unchanged viathe output interface 34 in step S6.

If a path object is contained in the extracted object, then thepath-painting operator attribute confirming unit 40 confirms whether astroking operator, e.g., an operator “S”, “s” or the like, is containedin the path object or not in step S3. If a stroking operator is notcontained in the path object, or in other words, if a filling operator“f”, F″ or the like, is contained in the path object, then thepath-painting operator attribute confirming unit 40 outputs the pagedescription data Dp unchanged via the output interface 34 in step S6.

If a stroking operator is contained in the path object, then the curvedpath construction operator confirming unit 42 confirms whether a curvedpath construction operator, e.g., an operator “c”, “v” or “y”, iscontained in the path object or not in step S4. If a curved pathconstruction operator is not contained in the path object, or in otherwords, if only an operator “m”, “l”, “h” or “re” is contained in thepath object, then the curved path construction operator confirming unit42 outputs the page description data Dp unchanged via the outputinterface 34 in step S6.

If a curved path construction operator is contained in the path object,then the path construction operator replacer 44 replaces the curved pathconstruction operator with a plurality of straight path constructionoperators by which the curved path construction operator isapproximated, in step S5. Then, in step S6, the path constructionoperator replacer 44 outputs page description data Dp′ where the curvedpath construction operator has been replaced with the straight pathconstruction operators, via the output interface 34.

In this manner, the page description data Dp that are supplied to thepage description data processing apparatus 14 are supplied as theunchanged page description data Dp or the page description data Dp′where the curved path construction operator has been replaced with thestraight path construction operators, to the RIP 16, as shown in FIG. 1.

A process of replacing a path construction operator (step S5 in FIG. 4)will be described in detail below with reference to a flowchart shown inFIG. 5. The process shown in FIG. 5 is carried out by the pathconstruction operator replacer 44.

First, variables are initialized. Specifically, a flag whose value iseither 0 or 1, a variable n whose value is a non-negative integer, and avariable t_(n) whose value is a real number ranging from 0 to 1, are setto 0 in step S51. The variables will be described later on whennecessary.

Then, it is determined whether the flag is 0 or not in step S52. Sincethe flag is 0 initially, Δt is set to 0.1 in step S53. The initial valueof Δt is not limited to 0.1, but may be a suitable value for subsequentcalculations to be described below.

If the flag is 1, Δt is set to Δt/2 in step S54 as will be described indetail later.

Then, values of A, B are calculated according to predeterminedexpressions in step S55. The meanings of A, B will be described belowwith reference to FIG. 6.

FIG. 6 is a diagram illustrative of a process of determining a polygonalpath which is approximated to a path represented by the cubic Beziercurve 50 shown in FIG. 3.

In FIG. 6, a straight line T indicated by the broken line is a linetangential to the cubic Bezier curve 50 at a point C_(n) (t=t_(n))thereon. Using y(t) according to the expression (2), the gradient of thestraight line T is expressed as y′(t_(n)). The value of y at a pointC_(n+1)(t=t_(n)+Δt) can thus be linearly approximated byA=y(t_(n))+y′(t_(n))Δt. The actual value of y at a pointC_(n+1)(t=t_(n)+Δt) is represented by B=y(t_(n)+Δt).

The value |A−B| means an approximation error that is produced when thecubic Bezier curve 50 is linearly approximated by the straight line Twhich passes through the point C_(n).

Then, it is determined whether |A−B|<Err (allowable error) is satisfiedor not in step S56. If the approximation error |A−B| is smaller than theallowable error Err, then the flag is set to 0 in step S57. Theallowable error Err may be set depending on the accuracy with which animage is reproduced. If the print 26 is produced by the printing press24 which has a resolution of 2400 dpi, then it is preferable to set theallowable error Err to a value equal to or smaller than a pixel sizewhich corresponds to 1200 dpi, which is one-half of the resolution of2400 dpi. In this manner, the process shown in FIG. 5 can beappropriately performed depending on the output resolution of theprinting press 24.

If the approximation error |A−B| is equal to or greater than theallowable error Err, then the flag is set to 1 in step S58, and controlgoes back to step S52. Since the flag has been set to 1, Δt is reset toΔt/2 in step S54, so that the approximating range based on the straightline T (see FIG. 6) is reduce to half. In this fashion, steps S52through S56 are repeated to gradually reduce the approximating rangebased on the straight line T until the approximation error |A−B| becomessmaller than the allowable error Err.

It is preferable to keep the approximation error within a predeterminedrange for stably maintaining the quality of images such as characterimages.

If the approximation error |A−B| is smaller than the allowable error Err(i.e., |A−B|<Err), the flag is set to 0 in step S57. Thereafter, it isdetermined whether t_(n)+Δt≧1 is satisfied or not in step S59. Ift_(n)+Δt≧1 is not satisfied, i.e., if t_(n)+Δt<1, then it is judged thatthe process of dividing the cubic Bezier curve 50 into line segments ofa polygonal path is not completed yet. Thereafter, a current point,i.e., a joint between line segments, is set (t_(n+1)=t_(n)+Δt), and anext current point (n=n+1) is searched for in step S60. Thereafter,control goes back to step S52, and steps S52 through S60 are repeated.

If t_(n)+Δt≧1 is satisfied, then it is judged that the process ofdividing the cubic Bezier curve 50 into line segments of a polygonalpath is completed over the interval of 0≦t≦1. Thereafter, an end point(t_(n)=1) is determined, and the number M (=n+1) of current points isstored in step s61.

Then, the M current points {C_(m)} (m=0 through M−1) are calculated instep S62. The coordinates (x, y) of the M current points can becalculated according to the expressions (1), (2).

Finally, a group of straight path construction operators is generated,and then the curved path construction operator is replaced with thegroup of straight path construction operators in step S63. Morespecifically, (M−1) straight path construction operators having startpoints C_(m) and end points C_(m+1) are generated. Thereafter, thesingle curved path construction operator is replaced with the (M−1)straight path construction operators (operator “l”). The process is thusended.

FIG. 7A shows a path having a line shape which is the same as the lineshape shown in FIG. 8A. FIG. 7B shows a polygonal path converted fromthe path shown in FIG. 7A by a page description data processing methodaccording to the present embodiment.

As shown in FIG. 7B, a curved path P_(C) (curved path constructionoperator O_(C)) is replaced with line segments P_(L1) through P_(L4) ofa polygonal path which correspond respectively to straight pathconstruction operators O_(L1) through O_(L4). The line segments P_(L1)through P_(L4) of the polygonal path reproduce the curved path P_(C)with accuracy.

FIG. 7C shows a stroked area 100 produced when the line segments P_(L1)through P_(L4) of the polygonal path shown in FIG. 7B are rasterized bythe RIP 16.

As shown in FIG. 7C, the stroked area 100 is expressed more stably thanthe stroked areas 4, 6 (see FIGS. 8C and 8E). As no curved pathconstruction operator is present in the object, the stroked area 100 iscompletely free of any line join style instability caused by therasterizing process.

As described above, immediately before the page description data arerasterized by the RIP 16, the curved path P_(C) is replaced with theline segments P_(L1) through P_(L4) of the polygonal path. Thus, thepage description data are prevented from leading to print difficultiesdue to the characteristics inherent in the printing press 24, thesoftware version of the RIP 16, etc. Thus, possible print difficultiesin a process of rasterizing image data representing a precipitous sharpline shape can be prevented. The present invention is particularlyeffective in outlining fonts.

While the page description data according to the PDF have been describedabove, the present invention is not limited to the PDF, but is alsoapplicable to other page description languages such as PostScript(registered trademark) of Adobe Systems and XPS (XML PaperSpecification).

Although a certain preferred embodiment of the present invention hasbeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. A page description data processing apparatuscomprising: an input interface configured to input page description datato be supplied to a printing press; an object computing unit including astructural analyzer for extracting an object in the page descriptiondata; an object path computing unit connected to receive a result fromthe object computing unit, and configured to confirm whether or not thepage description data contain an object for stroking a path bydetermining presence or absence of data representing a stroking operatorin the page description data; a curved path computing unit connected toreceive a result from the object path computing unit when it isdetermined that the page description data contains the object forstroking a path, and configured to confirm whether or not a curved pathconstruction operator corresponding to a curved path is contained in theobject by determining presence or absence of data representing anoperator representing a type of curve in the page description data; anda path construction operator replacement computing unit connected toreceive a result from the curved path computing unit when it isdetermined that curved path computing unit confirms the curved pathconstruction operator, and configured to determine line segments of apolygonal path which is approximated to the curved path of the curvedpath construction operator and replacing the curved path constructionoperator with a plurality of straight path construction operators whichcorrespond respectively to the line segments of the polygonal path,wherein the path construction operator replacement unit determines theline segments of the polygonal path such that an approximation errorwith respect to the curved path falls within a predetermined range setdepending on output resolution of the printing press, wherein the linesegments are determined in a serial manner, and the spacing of the linesegments is variable, said curved path construction operator determinesthe spacing based on an approximation difference between a value ofrespective straight path construction operator at a predetermined pointand value of the curved path being approximated at the predeterminedpoint; and further wherein in the path construction operator, theapproximation difference is calculated reiteratively when theapproximation difference is determined to be greater than apredetermined threshold, and when the approximation difference isgreater than the predetermined threshold, the spacing of a next linesegment approximation relative to a current line approximation becomesreduced for determining a next approximation difference until theapproximation difference is less than the predetermined threshold, andan output interface configured to output robustized page descriptiondata having been subjected to the particular processing sequence by theobject computing unit, the object path computing unit, the curved pathcomputing unit, and path construction operator replacement unit.
 2. Apage description data processing method for using a page descriptiondata processing apparatus checking if page description data to besupplied to a printing press has a certain attribute and for performinga particular processing sequence on the page description data having thecertain attribute, comprising: using one or more processors to perform:an object confirming step of executing the page description dataprocessing apparatus to confirm whether or not the page description datacontain an object for stroking a path by determining the presence orabsence of data representing a stroking operator in the data descriptiondata; an operator confirming step of executing the page description dataprocessing apparatus to confirm whether or not a curved pathconstruction operator corresponding to a curved path is contained in theobject by determining presence or absence of data representing a curvedpath construction operator in the page description data, when the pagedescription data contain the object for stroking a path; and an operatorreplacing step of executing the page description data processingapparatus to determine line segments of a polygonal path which isapproximated to the curved path of the curved path construction operatorand replace the curved path construction operator with a plurality ofstraight path construction operators which correspond respectively tothe line segments of the polygonal path, when the curved pathconstruction operator is contained in the object, wherein in theoperator replacing step, the line segments of the polygonal path isdetermined such that an approximation error with respect to the curvedpath falls within a predetermined range set depending on outputresolution of the printing press, wherein the line segments aredetermined in a serial manner, and the spacing of the line segments isvariable, said curved path construction operator determines the spacingbased on an approximation difference between a value of respectivestraight path construction operator at a predetermined point and valueof the curved path being approximated at the predetermined point,wherein the approximation difference is calculated reiteratively whenthe approximation difference is determined to be greater than apredetermined threshold, and when the approximation difference isgreater than the predetermined threshold, the spacing of a next linesegment approximation relative to a current line approximation becomesreduced for determining a next approximation difference until theapproximation difference is less than the predetermined threshold; andoutputting a display result based on the aforesaid steps.
 3. Anon-transitory computer-readable recording medium storing a program forenabling a computer to check an attribute of page description data to besupplied to a printing press and perform a processing sequence on thepage description data depending on the checked attribute, the programfurther enabling the computer to perform steps of: object confirmationincluding confirming whether or not the page description data contain anobject for stroking a path; curved path construction operatorconfirmation including confirming whether or not a curved pathconstruction operator corresponding to a curved path is contained in theobject, in a case where the object confirmation step has confirmed theobject; and path construction operator replacement including determiningline segments of a polygonal path which is approximated to the curvedpath of the curved path construction operator and replacing the curvedpath construction operator with a plurality of straight pathconstruction operators which correspond respectively to the linesegments of the polygonal path, in a case where the curved pathconstruction operator confirmation step has confirmed the curved pathconstruction operator, wherein the path construction operatorreplacement determines the line segments of the polygonal path such thatan approximation error with respect to the curved path falls within apredetermined range set depending on output resolution of the printingpress, wherein the line segments are determined in a serial manner, andthe spacing of the line segments is variable, said curved pathconstruction operator determining the spacing based on an approximationdifference between a value of respective straight path constructionoperator at a predetermined point and value of the curved path beingapproximated at the predetermined point, and wherein the approximationdifference is calculated reiteratively when the approximation differenceis determined to be greater than a predetermined threshold, and when theapproximation difference is greater than the predetermined threshold,the spacing of a next line segment approximation relative to a currentline approximation becomes reduced for determining a next approximationdifference until the approximation difference is less than thepredetermined threshold.
 4. The apparatus according to claim 1, whereinthe path construction operator replacement further determines whether asufficient number of line segments for approximating the curve have beendetermined.